1. Field of the Invention
The present invention relates to acicular alloy magnetic powder, a method for producing the magnetic powder and a magnetic recording medium comprising the magnetic powder. More particularly, it relates to acicular fine particle magnetic powder made of an alloy comprising iron and cobalt, a method for producing such fine particle magnetic powder and a magnetic recording medium comprising such fine particle magnetic powder and having improved corrosion resistance.
2. Description of the Related Art
Since metal iron base magnetic powder has larger coercive force and saturation magnetization than iron oxide base magnetic powder and is suitable for high density recording, it is now practical to be used for producing a magnetic recording medium.
Since the particle surfaces of metal iron base magnetic powder are very active and easily corroded, its handling is difficult, and further the magnetic recording medium comprising the metal iron base magnetic powder suffers from decrease of output characteristics under high temperature and high humidity conditions. This is apparent from the fact that the saturation magnetization of the metal iron base magnetic powder greatly decreases in several hours when the magnetic powder is placed in an atmosphere of 60.degree. C. and humidity of 90%.
To improve the corrosion resistance of the metal iron magnetic powder, it is proposed to use an alloy of iron with other metal such as cobalt whereby a passive state film is formed on the particle surface.
Standard methods for producing the alloy base magnetic powder include:
(1) reduction of a co-precipitated material prepared from an iron salt and a cobalt salt which are added to an aqueous solution of oxalic acid;
(2) thermal reduction of iron oxide particles on which surface cobalt is deposited;
(3) addition of a reducing agent to a solution comprising an iron salt and a cobalt salt;
(4) evaporation of metal in an inert gas to cause collision of the evaporated metal with the gas molecules; and
(5) reducing iron chloride and cobalt chloride both in vapor states in a mixture of hydrogen with nitrogen or argon to form metals.
In the method (1), control of the composition of the particles is difficult. In the method (2), since the cobalt compound is formed on the iron particles, it is difficult to maintain the acicular form. In the methods (3), (4) and (5), the produced magnetic powder is not acicular but in the form of chained beads and does not have satisfactory orientation.
To overcome the above problems, it is proposed to thermally reduce acicular goethite particles which contain cobalt and are prepared from alkaline aqueous suspensions of the iron salt and the cobalt salt.
Although the alloy base magnetic powder produced by the above method has better corrosion resistance than the conventional metal iron base magnetic powder, the content of cobalt in the powder does not exceed about 7% by weight and at such low cobalt content, satisfactory corrosion resistance cannot be achieved. The reason for this has not been made clear, but may be attributed to insufficient formation of the passive state on the particle surfaces because of shortage of cobalt in the metal magnetic powder.
Then, the present inventors thought that it would be necessary to protect the passive state film or to supply a sufficient amount of cobalt, and performed the following experiments.
To supply the sufficient amount of cobalt in the above method comprising thermally reducing the acicular goethite which contains cobalt, an excess amount of the cobalt salt was added to the aqueous suspension in order to increase the cobalt content in the produced magnetic powder. However, the particle shape or uniformity of the composition were disturbed, that is, the shape of goethite particle was deformed, or particles with irregular shapes were contained in the goethite powder. Therefore, the sufficiently large amount of cobalt cannot be introduced in the metal magnetic powder.
The present inventors investigated causes for such phenomena, and it is found that the problems will not be solved by the conventional methods in which the cobalt salt is added to the suspension during formation of the goethite powder. The reasons for this are as follows:
First, since the iron constituting the goethite is trivalent and is not equal to the valency of cobalt which is divalent, the iron ions and the cobalt ions cannot be freely exchanged. Second, the cobalt concentration in the aqueous suspension may control a growth rate of the goethite crystal. Third, since the shape of goethite particle determines the shape of metal magnetic powder particle through subsequent processing of the goethite powder, it is preferred that the cobalt ions which influence the growth rate of the goethite crystal are not present during the formation of the goethite particles.